1. Field of the Invention
The present invention relates to a subharmonic mixer, and more particularly to a subharmonic mixer capable of operating with a low-voltage power supply, reducing 1/f noise, and enhancing gain and linearity.
2. Description of the Prior Art
The direct conversion receiver is being actively studied as one of the receivers having a structure which can be built in a single chip. Since the direct conversion receiver can reduce external devices such as filters or the like as well as reducing the digital signal-processing load, the direct conversion receiver has a structure most suitable for fabrication of a single chip by use of a CMOS process facilitating the implementation of a digital circuit. Direct conversion receivers include a radio frequency (RF) direct conversion receiver for converting an RF signal into a base band signal, and an intermediate frequency (IF) direct conversion receiver for converting an RF signal into a specific IF signal and then converting the IF signal into a base band signal.
A mixer used for such a direct conversion receiver mixes a wireless frequency signal with a local oscillation signal (LO+) and a local oscillation signal (LO−) orthogonal to the local oscillation signal (LO+), thereby outputting two base band vector signals I and Q.
The subharmonic mixer, unlike a typical mixer using a CMOS Gilbert cell, forms plural stages serving as a switching stage, down-converting a frequency to a base band frequency through several stages. Such down-converting of a frequency through several stages can prevent the self missing of a local oscillator (LO self missing) leaking an LO signal into an input stage of a mixer through parasitic capacitance or the like.
FIG. 1 is a circuit diagram for showing a conventional subharmonic mixer used for a direct conversion receiver.
As shown in FIG. 1, the subharmonic mixer has a pair of amplification devices M11 and M12, and also has an amplification part 10 amplifying and supplying an input signal to a mixing part 20 and the mixing part 20 containing plural pairs of switches.
The mixing part 20 includes the first switching device group MA11, MA12, MB11, and MB12 for switching the amplification device M11 and the second switching device group MA21, MA22, MB21, and MB22 for switching the amplification device M12.
The switching devices MA11 and MB11 of the first switching device group form a cascode structure with the source and drain of the devices connected to each other, and the switching devices MA12 and MB12 also form a cascode structure. Further, the switching devices MA11 and MA12 and the switching devices MB11 and MB12 are connected in parallel, respectively. The switching devices MB11 and MB12 are supplied with local oscillation signals having a phase of 270° and a phase of 90°, respectively, and the switching devices MA11 and MA12 are supplied with local oscillation signals having a phase of 180° and a phase of 0°, respectively. Thus, the local oscillation signals input to the switching devices MA11 and MA12 have a phase difference of 180°, and the local oscillation signals inputted to the switching devices MB11 and MB12 have a phase difference of 180°. Since the first switching device group has two switching stages in the cascode structure, the first switching device group forms a frequency two times higher than a frequency of a local oscillation signal input to the gate of each switching device even though a frequency corresponding to a half a frequency for a general mixer is applied. Thus, the problems of dc-offset and LO self missing which occur in a conventional general mixer can be prevented since the local oscillation signal and the input signal have the same frequency.
The second switching device group brings out the same effect since it has the same structure as the first switching device group.
However, since such a subharmonic mixer has two switching stages and has a voltage drop occurring across each switching device, the subharmonic mixer has difficulties in operations at low voltages due to the voltage head room.
Since the switching devices M11 and M12 operate as amplifiers in the RF direct conversion receiver, the DC offset problem occurs as well as the non-linearity even though a high gain is obtained due to the amplifier characteristics. Further, each switching device produces less 1/f noise as smaller electric current is applied thereto, but requires electric current larger than a certain level for operations in the amplification part 10, so each switching device inevitably produces the 1/f noise. Since such 1/f noise increases as a frequency decreases from a high frequency to a low frequency, it is important to reduce the 1/f noise in the RF direct conversion receiver outputting a low-frequency base band signal.